1. Field of the Invention
The present invention relates to a camera having an auto-focussing device.
2. Related Background Art
Hitherto, as one type of focussing device for a camera, there is known a method in which an exit pupil of a photographic lens is divided into two areas by an optical system used to detect the focus. Two images of an object, each formed by rays passing through an aperture area, are received by photoelectric conversion element arrays (e.g., CCD sensor arrays). The focus state of the photographic lens is detected from outputs thereof, and the photographic lens is driven on the basis of the detection results.
In addition one form of usage of the camera is a photography state called "continuous shooting" in which shooting is effected repeatedly and continuously while the film is wound by an externally-attached motor drive unit or a built-in winder. In this state, a method is adopted in which the above-described focussing operation is carried out during each pause between the shooting of one frame and the winding of the film.
However, if, in the above-described continuous shooting state, an arrangement is made to delay the next shot until the in-focus state is obtained, and if an object for which focus control is difficult is being photographed, the shooting of the next frame is delayed by the time required by the focussing operation until that in-focus state is obtained. This results in very poor operational characteristics, in view of the fact that continuous shooting is often used to follow a moving object.
In addition, when the focus detection is effected by using an image storage-type sensor, such as a CCD, in a focus detection operation during focussing in the above-described continuous shooting, if the storage time is controlled in such a manner that the level of the stored image obtained by the sensor reaches an appropriate value, a longer storage time is required if the illumination is low, so that it is likely that the purpose of continuous shooting cannot be attained.